The present invention relates to a method of removing organic and inorganic sulfur compounds from coal and other carbonaceous combustible materials.
The recent energy crisis has increased the consumption of coal in the United States. However, there are many problems which need to be solved concerning the use of coal. The most important is environmental pollution caused by burning coal.
Most coal found in the United States contains from 0.5 up to 10 weight percent sulfur. When coal is burned, the sulfur is emitted as sulfur dioxide, which causes serious pollution of the atmosphere. Moisture in the atmopshere then combines with the sulfur dioxide to produce "acid rain".
Because of the adverse impact of large quantities sulfur from burning coal on the environment, it is necessary to substantially reduce the amount of sulfur which is released to the atmosphere. Only a small fraction of the available coal can be burned directly without violating current pollution control regulations. Thus methods must be developed to either decrease the amount of sulfur in the coal before it is combusted or to remove the sulfur from the flue gas. However, flue gas desulfurization is expensive because of the high cost of the capital equipment and the high cost of maintaining that equipment. Pre-combustion processes include conventional physical cleaning, chemical treatment, magnetic separation and coal conversion such as gasification and liquefaction. Most physical cleaning methods separate mineral impurities from coal on the basis of the differences in density of coal and mineral matter. This method only removes coarse mineral partices which are easily released, while leaving the finer particles in the coal. Coal gasification and liquefaction are not yet fully developed and are expensive. Magnetic separation again can remove only liberated particles but does remove some ash forming minerals in addition to some sulfur. Chemical cleaning methods, which can remove both organic and inorganic sulfur, are more effective than physical cleaning methods and are generally more economical than gasification and liquefaction.
Sulfur, in coal, may be classified into two general types, organic and inorganic. Organic sulfur is chemically bonded to the hydrocarbon matrix of coal and can only be removed by chemical means. Inorganic sulfur is present in coal as pyrite (FeS.sub.2) and in small amounts as a sulfate (generally, calcium sulfate or ferrous sulfate). Pyritic sulfur may range from 0.5 to 10.0 weight percent with individual particles of pyrite ranging from a few microns to a few inches in diameter. Large, liberated pyrite particles can be removed by hand or by various mechanical cleaning methods, but particles that are finely distributed in the coal matrix need first to be liberated by fine grinding.
A number of different processes have been developed for the removal of organic and inorganic sulfur from coal and other burnable carbonaceous materials and for reducing the ash content of these materials. One such method is known as the Gravimelt Process. The method is based on treating one part of finely powdered coal with approximately ten parts of a fused alkali (sodium hydroxide, potassium hydroxide, or a sodium hydroxide--potassium hydroxide mixture) at 300.degree.-400.degree. C. for 20-70 minutes. After removal of the coal floating on top of the melt and washing extensively with water to remove residual alkali and reaction products, substantial reductions in the sulfur content can be achieved. Subsequent washing of the treated coal with dilute sulfuric acid removes much of the mineral matter, leaving a product that is a relatively low-sulfur, low ash fuel. However, while the process is effective, significant amounts of organic sulfur remain in the coal, where it is burned and vented to the atmosphere as sulfuric dioxide.
In another process for removing sulfur, a slurry of finely divided coal in a solvent of methylchlorform, carbon tetrachloride or tetrachlorethylene and 30-70 weight percent water is prepared. Gaseous chlorine is bubbled through the slurry at 60.degree.-130.degree. C. and 0-60 psig from 45 to 90 minutes to oxidize the coal. The process will remove about 60% of total sulfur in the coal, removing about 50% of the organic sulfur and 70% of the pyritic sulfur. While the process is reasonably effective, about half of the organic sulfur remains in the coal. Furthermore, substantial amounts of residual chlorine remain in the cleaned coal which can produce highly corrosive combustion products upon burning.
What is needed is a process for the removal of organic sulfur from combustible carbonaceous material such as coal, which is at least as effective as are processes for removing inorganic sulfur from the coal.